RADIOACTIVITY ALONG THE 80TH MERIDIAN
493
1961 tests; this is in agreement with the reports that 85 to 90% of the
Sr in rainfall at Abingdon and Milford Haven in the United Kingdom*!
and at Westwood, N. J.°° during the spring of 1962 could be ascribed to
the 1961 U.S.S.R. test series,
In the May and June profile shown in Fig. 10, the pre-1961 debris
continued to exhibit the expected shape with higher Sr values in the
north as a result of the spring maximum there. The northern portion of
the 1961 U.S.S.R. Sr profile was also relatively unchanged; the curve
could not be extended south of the equator because of the influx there
of fresh debris from the 1962 Operation Dominic I tests, which started
April 25. The activity from the Christmas Island area arrivedat its
highest concentration at the high-altitude site at Chacaltaya, Bolivia,
as had occurred in past tests in the Pacific area.’:!! It was confined to
the relatively low latitudes of the southern hemisphere. As indicated
earlier, no significant quantity of this activity appeared at ground level
in the northern hemisphere.
During the July and August period, there was only a minor contribution of Sr from the U.S.S.R. 1962 tests to the northern-hemisphere
air, and, in general, patterns of Sr throughout the world were similar
to those of the previous period. In later periods it was not possible to
make a completely objective assignment of debris tothe various sources
because of the lack of sufficient analytical data to discriminate between the four possible sources of debris. For these last two periods,
values based on past experience were assumed for the pre-1961 “Sr
concentrations in the northern hemisphere. This permitted a resolution of the residual two-component mixture. The resulting profiles for
the November and December period are shown in Fig. 11; the September and October profiles, which are not shown, were similar.
At the end of 1962, the profiles of premoratorium "Sr and “Sr
from the U. S. 1962 tests were nearly identical in the southern hemisphere. Moreover, activity ratios indicated the effective date of forma-
tion of new debris at all these sites to be in June 1962. A stratospheric
origin for both components thus appears reasonable. In the northern
hemisphere, U.S.S.R. 1962 tests were the major contributor to the “Sr
levels, and dates of formation in September and October were ob-
tained. This is apparently a case where the direct tropospheric intro-
duction of “Sr was of greater significance than deposition from a wellcharged stratospheric source. The peak in the U.S.S.R. 1961 Sr
profile at Miami is due to the earlier arrival there of the start of the
1963 winter—spring subsidence of stratospheric debris.
CONCLUSIONS
This study of the fission-product content of the atmosphere has
covered an interesting period of the nuclear age and in the development